It’s useful to have an alt because some instances block other instance you might be interested in.
Also, performance and availability reasons. If lemmy.world or lemmy.ml or in your case beehaw.org goes down, you can just use your account on another instance as if nothing happened.
There’s also been situations like vlemmy.net which the admins seems to have decided to pull the plug with no warning and disappeared, then there’s lemmy.fmhy.ml whose domain just got cancelled among many other free ml domains and is/was down for an extended period of time.